1. Field of the Invention
The present invention relates to a switching regulator.
2. Description of the Related Art
As a switching regulator, there have been known methods of adjusting a maximum duty ratio as shown in a circuit diagram of FIG. 3. In FIG. 3, a switching regulator circuit includes: an error amplifier 10 for comparing a voltage obtained by dividing an output voltage Vout of the switching regulator by resistances 2 and 3 and a voltage of a reference voltage source 4; and a comparator 7 for making a comparison among an output voltage Vr of the error amplifier 10, an output voltage of an oscillator 1 for generating a triangular wave, and a voltage Vp divided by resistances 5 and 6.
An output voltage EXT of the comparator 7 is used to drive a switching transistor of the switching regulator. As shown in FIGS. 4A and 4B, the comparator 7 compares an output of the oscillator 1 and a value of the lower one of the voltages Vr and Vp.
In other words, even if the voltage Vout drops and the voltage Vr reaches a high voltage level, as shown in FIG. 4B, a duty ratio of the voltage EXT by no means exceeds a maximum duty ratio defined on the basis of the voltage Vp.
Further, in some conventional methods, one more oscillator circuit that oscillates at the same frequency is adopted to adjust the maximum duty ratio (see, for example, JP 2000-217340 A).
In addition, in some conventional methods, a circuit for outputting a voltage depending on a power source voltage is used to adjust the maximum duty ratio (see, for example, JP 11-235025 A).
In the conventional methods of adjusting the maximum duty ratio, however, the voltage Vp needs to be adjusted using the two resistances 5 and 6, which causes a problem in that the two resistances occupy a larger packaging area.